Folded conformations of antigenic peptides from riboflavin carrier protein in aqueous hexafluoroacetone

Riboflavin carrier protein (RCP) plays an important role in transporting vitamin B2 across placental membranes, a process critical for maintenance of pregnancy. Association of the vitamin with the carrier protein ensures optimal bioavailability, facilitating transport. The conformations of three antigenic peptide fragments encompassing residues 4-23 (N21), 170-186 (R18), and 200-219 (Y21) from RCP, which have earlier been studied as potential leads toward a synthetic peptide-based contraceptive vaccine, have been investigated using CD and NMR spectroscopy in aqueous solution and in the presence of the structure-stabilizing cosolvent hexafluoroacetone trihydrate (HFA). In aqueous solution at pH 3.0, all three peptides are largely unstructured, with limited helical population for the peptides R18 and Y21. The percentage of helicity estimated from CD experiments is 10% for both the peptides. A dramatic structural transition from an unstructured state to a helical state is achieved with addition of HFA, as evidenced by intensification of CD bands at 222 nm and 208 nm for Y21 and R18. The structural transition is completed at 50% HFA (v/v) with 40% and 35% helicity for R18 and Y21, respectively. No structural change is evident for the peptide N21, even in the presence of HFA. NMR analysis of the three peptides in 50% HFA confirms a helical conformation of R18 and Y21, as is evident from upfield shifts of CαH resonances and the presence of many sequential NH/NH NOEs with many medium-range NOEs. The helical conformation is well established at the center of the sequence, with substantial fraying at the termini for both the peptides. An extended conformation is suggested for the N21 peptide from NMR studies. The helical region of both the peptides (R18, Y21) comprises the core epitopic sequence recognized by the respective monoclonal antibodies. These results shed some light on the issue of structure and folding of antigenic peptides

The structural basis of β2 integrin intra-cellular multi-protein complexes

In multicellular organisms, cell adhesion is a pivotal physiological process which is essential for cell-cell communications, cell migration, and interactions with extracellular matrix. Integrins, a family of large hetero-dimeric type I membrane proteins, are known for driving cell adhesion functions. Among 24 different integrins, four β2 integrins, αL β2, αM β2, αX β2 and αD β2, are specific for cell adhesion and migration of leukocytes. Many cytosolic proteins interact with short cytosolic tails (CTs) of β2 and other integrins which are essential in bi-directional signaling processes. Further, phosphorylation of CTs of integrins regulates binding of intra-cellular proteins and signaling systems. In this review, recent advances in structures and interactions of multi-protein complexes of integrin tails, with a focus on β2 integrin, and cytosolic proteins are discussed along with a proposed future direction.Ministry of Education (MOE)Submitted/Accepted versionStudies of integrins are support by grants from Biomedical Research Council (BMRC), Singapore and Ministry of Education (MOE), Singapore

NMR Structures and Interactions of Antimicrobial Peptides with Lipopolysaccharide: Connecting Structures to Functions

Antimicrobial peptides (AMPs) establish the first line of host defense mechanism against invading microorganisms including bacteria, viruses, fungi and parasites. In recent years, emergence and spread of antibiotic resistance bacterial pathogens have dawn considerable interest in investigations of AMPs. The ability of AMPs to exert lethality against multiple drug-resistant (MDR) bacteria has incited promising avenues for antibiotic development. As a mode of action, most AMPs perturb the membrane organization of bacterial cells. The outer membrane lipopolysaccharide (LPS) of Gram-negative bacteria establishes a superior permeability barrier, in contrast to the peptidoglycan layer of Gram-positive bacteria. Due to LPS barrier, development of antibiotics for drug resistant Gram- negative bacteria are more complicated, with only fewer compounds in the pipeline. Recent studies have demonstrated that LPS actively regulate mode of action of AMPs on the lethality of Gram-negative bacteria. LPS, also known as endotoxin, is the primary agent for septic shock syndromes in intensive care unit killing over 120,000 people in the USA. Currently, anti-sepsis therapies are greatly lacking. Therefore, LPS has been considered as a target for the development of antimicrobial and antisepsis drugs. In recent and past few years, 3-D structures and interactions of a number of AMPs have been determined in complex with LPS micelles. These studies have generated molecular insights towards mode of action and synergistic activity of AMPs in the outer membrane. In this review, atomic resolution structures and interactions of potent AMPs with LPS are discussed providing novel insights of their mode of action.MOE (Min. of Education, S’pore)Accepted versio

Special Issue “Selected Papers from the 8th Asia-Pacific NMR (APNMR) Symposium: Recent Advances in NMR Spectroscopy”

Asia-Pacific NMR (APNMR) has been an important scientific event in the region, engaging a large number of NMR scientists from academia and industries [...

NMR structures and localization of the potential fusion peptides and the pre-transmembrane region of SARS-CoV: Implications in membrane fusion

AbstractSevere acute respiratory syndrome-associated coronavirus (SARS-CoV) poses a serious public health hazard. The S2 subunit of the S glycoprotein of SARS-CoV carries out fusion between the virus and the host cells. However, the exact mechanism of the cell fusion process is not well understood. Current model suggests that a conformational transition, upon receptor recognition, of the two heptad core regions of S2 may expose the hydrophobic fusogenic peptide or fusion peptide for membrane insertion. Three regions of the S2 subunit have been proposed to be involved in cell–cell fusion. The N-terminal fusion peptide (FP, residues 770–788), an internal fusion peptide (IFP, residues 873–888) and the pre-transmembrane region (PTM, residues 1185–1202) demonstrated interactions with model lipid membranes and potentially involved in the fusion process. Here, we have determined atomic resolution structures of these three peptides in DPC detergent micelles by solution NMR. FP assumes α-helical conformation with significant distortion at the central Gly residues; enabling a close packing among sidechains of aromatic residues including W, Y and F. The 3-D structure of PMT is characterized by a helix–loop–helix with extensive aromatic interactions within the helices. IFP adopts a rather straight α-helical conformation defined by packing among sidechains of aromatic and aliphatic residues. Paramagnetic spin labeled NMR has demonstrated surface localization of PMT whereas FP and IFP inserted into the micelles. Collectively, data presented in this study will aid in understanding fusion mechanism of SARS-CoV

De novo-designed β-sheet heme proteins

The field of de novo protein design has met with considerable success over the past few decades. Heme, a cofactor has often been introduced to impart a diverse array of functions to a protein, ranging from electron transport to respiration. In nature, heme is found to occur predominantly in α-helical structures over β-sheets, which has resulted in significant designs of heme-proteins utilizing coiled coil helices. By contrast, there are only a few known β-sheet proteins that bind heme and designs of β-sheets frequently result in amyloid-like aggregates. This review reflects on our success with designing a series of multi-stranded β-sheet heme binding peptides that are well folded both in aqueous and membrane-like environments. Initially, we designed a β-hairpin peptide that self-assembles to bind heme and performs peroxidase activity in membrane. The β-hairpin was optimized further to accommodate a heme binding pocket within multi-stranded β-sheets for catalysis and electron transfer in membranes. Furthermore, we de novo designed and characterized β-sheet peptides and mini-proteins soluble in aqueous environment capable of binding single and multiple hemes with high affinity and stability. Collectively, these studies highlight substantial progress made towards the design of functional β-sheets.Ministry of Education (MOE)Accepted versionAS would like to thank Ministry of Education, Singapore and Nanyang Technological University for Graduate Research Scholarship

Designed di-heme binding helical transmembrane protein

De novo designing of functional membrane proteins is fundamental in terms of understanding the structure, folding, and stability of membrane proteins. In this work, we report the design and characterization of a transmembrane protein, termed HETPRO (HEme-binding Transmembrane PROtein), that binds two molecules of heme in a membrane and catalyzes oxidation/reduction reactions. The primary structure of HETPRO has been optimized in a guided fashion, from an antimicrobial peptide, for transmembrane orientation, defined 3D structure, and functions. HETPRO assembles into a tetrameric form, from an apo dimeric helical structure, in complex with cofactor in detergent micelles. The NMR structure of the apo HETPRO in micelles reveals an antiparallel helical dimer that inserts into the nonpolar core of detergent micelles. The well-defined structure of HETPRO and its ability to bind to heme moieties could be utilized to develop a functional membrane protein mimic for electron transport and photosystems